The present invention relates, in general, to automotive fuel leak detection methods and systems and, in particular, to a temperature correction approach to automotive evaporative fuel leak detection.
Automotive leak detection systems can use either positive or negative pressure differentials, relative to atmosphere, to check for a leak. Pressure change over a given period of time is monitored and correction is made for pressure changes resulting from gasoline fuel vapor.
It has been established that the ability of a leak detection system to successfully indicate a small leak in a large volume is directly dependent on the stability or conditioning of the tank and its contents. Reliable leak detection can be achieved only when the system is stable. The following conditions are required:
a) Uniform pressure throughout the system being leak-checked;
b) No fuel movement in the gas tank (which may results in pressure fluctuations); and
c) No change in volume resulting from flexure of the gas tank or other factors.
Conditions a), b), and c) can be stabilized by holding the system being leak-checked at a fixed pressure level for a sufficient period of time and measuring the decay in pressure from this level in order to detect a leak and establish its size.
The method and sensor or subsystem according to the present invention provide a solution to the problems outline above. In particular, an embodiment of one aspect of the present invention provides a method for making temperature-compensated pressure readings in an automotive evaporative leak detection system having a tank with a vapor pressure having a value that is known at a first point in time. According to this method, a first temperature of the vapor is measured at substantially the first point in time and is again measured at a second point in time. Then a temperature-compensated pressure is computed based on the pressure at the first point in time and the two temperature measurements.
According to another aspect of the present invention, the resulting temperature-compensated pressure can be compared with a pressure measured at the second point in time to provide a basis for inferring the existence of a leak.
An embodiment of another aspect of the present invention is a sensor subsystem for use in an automotive evaporative leak detection system in order to compensate for the effects on pressure measurement of changes in the temperature of the fuel tank vapor. The sensor subsystem includes a pressure sensor in fluid communication with the fuel tank vapor, a temperature sensor in thermal contact with the fuel tank vapor, a processor in electrical communication with the pressure sensor and with the temperature sensor and logic implemented by the processor for computing a temperature-compensated pressure based on pressure and temperature measurements made by the pressure and temperature sensors.